Residential construction has experienced steady change over the past several decades including larger homes, different home geometries, and a different mix of construction materials. Some of these changes are directly related to the increased use of light-weight engineered floor systems comprised of structural engineered wood I-joists replacing solid-sawn 2″×10″ wood joists. Wood I-joists are made of upper and lower horizontal components called the upper and lower flanges and a vertical section called a ‘web,’ which is between the flanges. The flanges are typically made from laminated veneer lumber or laminated structural lumber. The web is typically made from oriented strand board, a structural panel made with layers of thin, rectangular strands of wood produced by a cutting machine called a strander. The wood strands are mixed with adhesives and glued under heat and pressure to the desired panel thickness.
There has been on-going debate regarding the safety of these engineered wood-based systems under fire conditions, specifically pertaining to occupant evacuation and emergency responder entry. Under fire conditions, due to burn-through of the web, I-joists tend to lose strength quickly, leading to joist failure and floor collapse. Recent building code revisions (specifically the 2012 and 2015 IRC code revisions, 501.3 and 302.13 respectively) mandate the use of sprinkler systems or other approved floor assemblies demonstrating equivalent fire performance compared with 2″×10″ solid wood as measured by ASTM E119 and/or ASTM E-84 test results as well as the ICC Evaluation Service approval criteria for coatings.
A flame retardant coating is one of the easiest and most efficient ways to protect materials against fire. These coatings do not cause chemical modification of the substrate, but rather the formation of a protective layer which alters the heat flux to the substrate and can inhibit its thermal degradation, ignition, or combustion. Intumescent coatings are commonly used as flame retardants. Intumescent coatings expand under the influence of heat to form a multicellular charred layer which acts as an insulating barrier. The intumesced char can expand up to 50 times the original thickness of the applied coating. It can insulate the substrate and can give additional time before the substrate reaches a critical temperature at which it loses its load bearing capacity. This extra time can allow for evacuation of occupants in the building and a chance for emergency responders to control the fire. However, many intumescent coatings are designed to be used with steel substrates and may not be suitable for use with wood. Therefore, intumescent coatings suitable for use with I-joists and other wood substrates which meet the required fire retardancy standards and approval criteria, would be desirable.